A microgrid is a localized grouping of electricity generation, energy storage, and loads that normally operates connected to a traditional centralized grid (macrogrid) via a PCC. This single point of common coupling with the macrogrid can be disconnected, islanding the microgrid. Microgrids are part of a structure aiming at producing electrical power locally from many small energy sources, distributed generators (DGs). In a microgrid, a DG is connected via a converter which controls the output of the DG, i.e. the current injected into the microgrid.
A microgrid (in grid connected mode, i.e. connected to the macrogrid) supplies the optimized or maximum power outputs from the connected DG sites and the rest of the power is supplied by the macrogrid. The microgrid is connected to the macrogrid at a PCC through a controllable switch. This grid connection is lost during grid fault and the microgrid is islanded.
During islanding, there is a risk of imbalance in the microgrid due to the loss of power import from grid as well as loss of voltage control by the grid. For voltage control it is required to change control mode of the DGs. The power balancing is solved by fast storage action and immediate load shedding schemes.
In an AC microgrid, the frequency is the same everywhere in steady state while voltage may differ depending on the power flow. However, in a microgrid with a continuous change in DG output, load switching and low inertia, there is continuous frequency and voltage fluctuation to a small scale. The deviations are larger during large transients (i.e. DG fault etc.). Frequency and voltage stability relates to minimum oscillations and overshoot with ability to come back to initial value (or any other steady state value within acceptable deviation) after a disturbance.
A microgrid with multiple DGs and loads requires several switches at different level to connect and disconnect to the main power grid as well as different network parts within the microgrid. The automatic disconnections by these switches are aimed for system protection. Planned connections and disconnections are required for optimized operation ensuring power balance and resynchronization maintaining acceptable system dynamics in voltage, frequency and power oscillations. Before reconnection of a network to the microgrid resynchronization with voltage magnitude, phase angle and frequency matching is performed to ensure stability of the microgrid at reconnection.
A microgrid may have more than one PCC. In that case, power transfer may still occur over one of the PCCs if the microgrid is islanded at another of the PCCs. Typically, the power transfer will then increase substantially over the PCC which is still connected to compensate for the islanding. Similarly, if an islanded PCC is reconnected, then the power transfer over the other, already connected, PCC will typically drop substantially.